Bored tunnels using a Tunnel Boring Machine (TBM) are typically lined with a permanent concrete lining material. Concrete liners can be either cast in place or can be precast segments joined together with the gaps between the segments being filled with a compressible gasket for precast concrete segments. The selection of the lining material is generally based on the tunnel function and the nature of the rock or soil through which the tunnel is bored or dug.
In regular geotechnical formations, either soft ground or hard, there is no risk of deformation of the tunnel. However, in time dependent rock formations (swelling rock), the deformations are significant and forces from the rock could damage the concrete liner over a period of time. In such cases, there is a time dependent deformation of the rock that can, if not accommodated by the tunnel lining, result in a reduction of the tunnel diameter and potential damage to the tunnel liner. Time dependent deformation is movement that occurs after the initial ground disturbance that results from tunneling. There are two distinct mechanisms that result in TDD, swelling and squeezing, although they may occur simultaneously and one may lead to the other. Swelling is the time dependent volume increase of the ground and squeezing is the time dependent shearing of the ground. Both phenomena lead to inward movement of the tunnel perimeter. The time dependent deformation generally occurs over a period of about 3 months to a few years after which time the pressures and forces are balanced and thus relieved.
In the past, in such time dependent deformation rock formations, typically the tunnel liners were cast in place liners constructed after 3 months. In such situations a temporary liner would be installed. This could result in significantly increased costs of the tunnel construction as a result of the delay in installation of the final liner.
It has been proposed in such situations to use precast tunnel liner segments where the tunnel is bored to a larger diameter than the finished diameter and to utilize a compressible mortar in the gap between the tunnel liner and the tunnel wall. One such compressible mortar is described in EP Patent No. 2,790,624 issued Nov. 28, 2006, to Hochtief Construction AG. This is also described by B. Billing et. al., Deco Grout—Innovative Grout to cope with rock deformations in TBM tunneling, in Underground Space—the 4th Dimension of Metropolises, Bartak et. al., eds. (2007), pages 1487-1492. As described in theses references, the grout utilizes expanded polystyrene beads along with foam to achieve a grout being capable of being compressed to about 50% of its initial volume. In the studies they tested expanded clays but rejected them as not being suitable. Also, they developed the grout independent of the actual forces or pressures as they state that these can only be calculated in advance with great difficulty and thus they attempted to develop a grout with a wide envelop of compressibility.
There thus remains a need for a compressible grout mix which can be formulated to provide the optimum compressibility for the specific application.